Zoom finder

ABSTRACT

A zoom finder comprising a system for forming the image of a measuring field frame and a vari-focal system in the order from the object side and arranged that the apparent size of the measuring field frame varies at the time of zooming.

BACKGROUND OF THE INVENTION

(a) Field of the invention:

The present invention relates to a zoom finder.

(b) Description of the prior art:

Known zoom finders are arranged that the apparent size of the measuringfield frame thereof does not vary at the time of zooming. Therefore,when a known zoom finder is used with an active-type automatic focusingcamera, the measuring field indicated on an object by the measuringfield frame becomes large in the wide position and small in theteleposition though the size of the measuring beam spot on the objectdoes not vary.

In an active-type automatic focusing camera, it is conventional toutilize a projected beam of light to illuminate an object to bephotographed. A detector lens is employed to measure the lens upondetection of the light reflected to provide data for automaticallyfocusing the camera lens. If the light from the projector does notcorrectly fall on the object, the distance will not be accuratelymeasured so that the auto focus device will not operate to provide aclear picture. In a camera provided with a zoom finder where the finderoperates between a wide angle and a telephoto position, the focusingbeam light from the project lens, called the "measuring beam spot", caneasily be improperly located when a shift is made between the side angleand telephoto positions of the zoom finder.

Thus, in case that the size of the measuring field frame is adjusted tothe size of the measuring beam spot in the state of the wide position,when photographing two persons who stand side by side for example, itapparently looks as if the measuring beam passes through the spacebetween the two persons, when zoomed to the teleposition, in spite ofthe fact that the measuring beam is virtually striking upon the twopersons sufficiently. Hence, to focus on the object accurately, it isunavoidable to put either of the two persons into the measuring fieldframe and, consequently, a photograph is often taken in such state thatone of the two persons comes to a position near the center of the fieldin spite of the fact that it is actually possible to take a photographby putting the two persons to the central portion of the field in a wellbalanced state.

On the other hand, in case that the size of the measuring field frame isadjusted to the size of the measuring beam spot in the state of theteleposition, it apparently looks, when zoomed to the wide position, asif the measuring beam is sufficiently striking upon the two persons inspite of the fact that the measuring beam is virtually passing throughthe space between the two persons and, as a result, the photograph oftenbecomes out of focus.

It will be apparent, therefore, that in a circumstance where a user isphotographing two objects, for example, two persons standing apart fromeach other but side by side, the measuring field frame, which is thefocusing frame, is so adjusted as to be coincident with the diameter ofthe measuring beam spot at a wide angle position so that the focusingframe will overlap or cover both of the objects. In this condition, thelight beam emitted from the camera will fall on both of the objects andit is therefore possible for the camera to correctly measure thedistance to the objects. However, when these objects are to bephotographed at the tele position by zooming the lens system from thewide angle position to the telephoto position, the finder will also beset at the telephoto position and the images of the objects observedthrough the finder will be enlarged and the space between the objectswill increase in width. However, in the conventional camera of thistype, the image size of the measuring field frame remains unchangedsince the actual distance from the camera to the objects does not vary.Accordingly, the light beam for measuring the distance will still fallon both of the objects in the same manner as when the focusing finder isadjusted to the wide angle position. However, many photographers,particularly those of limited experience for whom these cameras arespecifically designed, have tended to misjudge the location of the lightbeam distance measuring spot as it appears that it does not fall on bothof the objects when the finder is zoomed to the telephoto position. Inthis circumstance, many users will shift the camera so that the lightbeam spot will fall on one of the objects which procedure can result inan unbalanced and therefore less than satisfactory photograph. On theother hand, when the focusing frame is adjusted in the telephotoposition and then switched to the wide angle position through thezooming operation, the image of the measuring field frame, the focusingframe, is actually located between the two objects in the view of theuser while in fact the beam is actually illuminating both objects sothat the camera will measure the distance correctly and properly focusthe camera. Also in this case, when the user switches from the telephotoposition to the wide angle position, again, the actual distance betweenthe camera and the objects is unchanged so that a correct measureddistance will be effected. Unfortunately, when the measuring field frameis adjusted with the light beam spot in the telephoto position and thenswitched from the telephoto position to the wide position by zooming thelens system, the image of the measuring field frame, the focusing frame,is actually located between the object but since the focusing frame islocated so as to cover both of the objects the camera will be capable ofcorrectly measuring the distance but, as noted above, the user is likelyto misjudge the operation of the camera.

It is usually the condition where the zooming is effected from thetelephoto to the wide angles that errors effected by the user occursince, in this circumstance, the measuring light beam only apparently isin the correct position for measuring distance when in fact, in thecircumstance where two people are standing side by side the measuringlight beam will fall between.

A zoom finder disclosed in Japanese published examined patentapplication No. 9389/58 is known as a zoom finder whose lens system hasa distribution of refractive power resembling the distribution ofrefractive power in the preferred embodiment of the zoom finderaccording to the present invention, in other words, a zoom finder whoselens system forms a Galilean telescope comprising three lens units,i.e., a positive, negative and positive lens units. However, in case ofsaid known zoom finder, any consideration is not given to indication ofa measuring field frame and/or measuring beam spot into the field and,therefore, any solution is not given to the afore-mentioned disadvantageof known zoom finders.

SUMMARY OF THE INVENTION

It is, therefore, a primary object of the present invention to provide azoom finder arranged that the size of the image of the measuring fieldframe varies at the time of zooming.

To attain the above-mentioned object, the zoom finder according to thepresent invention is arranged to comprise a system for forming the imageof the measuring field frame, and a vari-focal system, in the order fromthe object side, and is thereby arranged that the apparent size of themeasuring field frame varies when the zoom finder is zoomed by means ofthe vari-focal system.

For the zoom finder according to the present invention, it is preferableto arrange that said system for forming the image of the measuring fieldframe has negative refractive power and said vari-focal system comprisesa lens unit which has positive refractive power and which is located onthe object side therein. Besides, a surface on which the meauring fieldframe is printed is arranged in front of said lens unit having positiverefractive power, and one of surfaces in said system for forming theimage of the measuring field frame is arranged as a semitransparentmirror so that the image of the measuring field frame is formed. Inother words, the surface on which the measuring field frame is printedis provided between said semitransparent mirror and said lens unithaving positive refractive power.

Besides, it is preferable to arrange that a subsystem (one lens unit ora plural number of lens units), which is located in rear of said lensunit having positive refractive power in the vari-focal system, hasnegative refractive power as a whole.

The zoom finder according to the present invention comprises, forexample, a first lens unit G₁ having negative refractive power, a secondlens unit G₂ having positive refractive power, and a third lens unit G₃having negative refractive power in the order from the object side asshown in FIGS. 1 and 2. The first lens unit G₁ is arranged to serve as asystem for forming the image of the measuring field frame and to formthe image of the measuring field frame at the position of the rear focalpoint of the first lens unit G₁. The second lens unit G₂ is the positivelens unit in the vari-focal system and is arranged to chiefly performthe vari-focal action. To arrange that the displacement of the imagepoint to be caused by the vari-focal action of the second lens unit G₂becomes small, the imaging magnification range of the second lens unitG₂ from the wide position to the tele position (from β_(2W) to β_(2T) )of the second lens unit G₂ is selected in the range of around -1/√R toaround -√R where reference symbol R represents the vari-focal ratio. Theangular magnification at the wide angle position may be represented byγ_(W) and that at the telephoto position as γ_(T) . Then, the vari-focalratio R is expressed as R=γ_(T) /γ_(W) . The third lens unit G₃ isarranged to have negative refractive power as a whole and to be moved atthe time of zooming so that the image formed by the first and secondlens units will be observed with a suitable diopter. In other words, thesecond lens unit G₂ and third lens unit G₃ constitute a so-calledvari-focal system and, out of them, the third lens unit G₃ may bearranged to have the function as a compensator.

In the zoom finder according to the present invention described so far,the first lens unit G₁ is located at a position where the heights ofprincipal rays are high compared with the heights of paraxial rays and,therefore, the first lens unit G₁ has large influence on offaxialaberrations, i.e., astigmatic difference, curvature of field,distortion, etc. Therefore, to correct said offaxial aberrations, it ispreferable to arrange that the first lens unit G₁ is provided with asurface which is concentric with the stop and a surface on whichprincipal rays are incident at comparatively large angles and, moreover,a suitable surface in the first lens unit G₁ is formed as an asphericalsurface.

The second lens unit G₂ is located at a position where the heights ofparaxial rays are high compared with the heights of principal rays.Therefore, it tends to cause spherical aberration which is paraxialaberration and, moreover, said spherical aberration varies at the timeof zooming. To solve the above-mentioned problem, for the second lensunit G₂ , it is preferable to use a glass material with a highrefractive index so that radii of curvature of respective surfacesbecome large and, at the same time, to arrange respective surfaces sothat the incident angles of paraxial rays on those surfaces becomesmall.

The third lens unit G₃ is located at a position where both of theheights of principal rays and heights of paraxial rays are low, but itis possible to control the value of spherical aberration by means of thethird lens unit G₃.

The variation of the diopter to be caused by zooming can be eliminatedby moving said third lens unit G₃ as described before. In that case, itis also possible to further divide the third lens unit G₃ into twosubunits and to move the lens (subunit) on the front side only so as toeliminate said variation of the diopter. The diopter can be keptconstant also when the first lens unit G₁ is moved by keeping the thirdlens unit G₃ fixed. As another method for keeping the diopter constant,the second lens unit G₂ may be divided into two subunits and either ofthem may be moved by keeping both of the first lens unit G₁ and thirdlens unit G₃ fixed. In that case, it is possible to perform zooming inthe state that the diopter is kept constant when the second lens unit G₂is moved by varying the airspace between said two subunits. As the thirdlens unit G₃ serves also as an eyepiece, it is preferable to arrangethat at least the lens (subunit) thereof on the eye point side is notmoved.

By arranging as described so far, it is possible to obtain an excellentzoom finder which makes it possible to attain the object of the presentinvention and, at the same time, which is arranged that aberrations arecorrected favourably.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 respectively show sectional views of a preferredembodiment of the zoom finder according to the present invention in thewide position and teleposition;

FIG. 3 shows a coordinate system illustrating the shape of theaspherical surface adopted in said embodiment of the present invention;and

FIGS. 4 and 5 respectively show graphs illustrating aberration curves ofsaid embodiment of the present invention in the wide position andteleposition.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Now, the preferred embodiment of the zoom finder according to thepresent invention described so far is shown below. Said preferredembodiment of the present invention has the lens configuration as shownin FIGS. 1 and 2. Out of them, FIG. 1 shows said embodiment in the wideposition and FIG. 2 shows said embodiment in the teleposition. Saidembodiment has the numerical data shown below.

    ______________________________________                                        r.sub.1 = ∞                                                             d.sub.1 = 1        n.sub.1 = 1.6968                                                                         ν.sub.1 = 5.55                               r.sub.2 = 26.338                                                              d.sub.2 = 7.9                                                                 r = -119.226                                                                  d.sub.3 = 1.1      n.sub.2 = 1.50                                                                           ν.sub.2 = 57.5                               r.sub.4 (aspherical surface)                                                  d.sub.4 = 1.16                                                                r.sub.5 = ∞ (measuring field frame)                                     d.sub.5 = 21.006˜6.397                                                  r.sub.6 = 70.851                                                              d.sub.6 = 3.82     n.sub.3 = 1.7725                                                                         ν.sub.3 = 49.6                               r.sub.7 = -70.731                                                             d.sub.7 = 12.81                                                               r.sub.8 = 39.115                                                              d.sub.8 = 3        n.sub.4 = 1.7725                                                                         ν.sub.4 = 49.6                               r.sub.9 = -206.024                                                            d.sub.9 = 1.033˜16.433                                                  r.sub.10 = -88.286                                                            d.sub.10 = 1       n.sub.5 = 1.50                                                                           ν.sub.5 = 57.5                               r.sub.11 = 28.630                                                             d.sub.11 = 1.58                                                               r.sub.12 = -101.085                                                           d.sub.12 = 1.01    n.sub.6 = 1.51633                                                                        ν.sub.6 = 64.1                               r.sub.13 = ∞                                                            d.sub.13 = 15                                                                 r.sub.14 (eye point)                                                          ______________________________________                                    

In the numerical data shown in the above, reference symbols r₁ throughr₁₄ respectively represent radii of curvature of respective lenssurfaces, reference symbols d₁ through d₁₃ respectively representthicknesses of respective lenses and airspaces between respectivelenses, reference symbols n₁ through n₆ respectively representrefractive indices of respective lenses, and reference symbols ν₁through ν₆ respectively represent Abbe's numbers of respective lenses.

In said embodiment, the method to form the image of the measuring fieldframe is adopted in the same way as Albada finder, and the measuringfield frame is printed on a film and is located at the position of r₅.Besides, a semitransparent mirror is provided to the surface r₂ andserves to form the image of the measuring field frame at the position ofthe rear focal point of the first lens unit G₁. As another method, itmay be arranged that the semitransparent mirror is obliquely provided inthe space between the first lens unit G₁ and second lens unit G₂ and theimage of the measuring field frame is introduced onto the optical axisof the finder optical system.

The surface r₄ is arranged as an aspherical surface which is expressedby the formula shown below when the coordinate system is set as shown inFIG. 3 (Where the optical axis is traced as the x axis). ##EQU1##

In case of the preferred embodiment, r₄ =52.996 and P₄ =0.0009 (where,reference symbol r₄ represents the radius of curvature of the vertexportion of the aspherical surface, and reference symbol P₄ representsthe coefficient of cone).

Aberration curves of said embodiment in the wide position andteleposition are as shown in FIGS. 4 and 5 respectively.

By the arrangement described so far, the zoom finder according to thepresent invention makes it possible to uniformly maintain the relationbetween the sizes and positions of the image of the measuring fieldframe and measuring beam spot independently of the state of zooming.Besides, as the zoom finder according to the present invention isarranged to comprise a negative, positive and negative lens units, it ispossible to arrange that a space for accommodating the system forforming the image of the measuring field frame is provided in front ofthe vari-focal system in spite of the fact that the number of lensesconstituting the zoom finder is small.

I claim:
 1. A zoom finder comprising:a system for forming an image of ameasuring field frame, said system including a semitransparent mirror;and a vari-focal lens system, said measuring field frame forming systemand said vari-focal lens system being formed in that order from theobject side of the zoom finder.
 2. A zoom finder according to claim 1wherein said system for forming the image of the measuring field framehas negative refractive power, and said vari-focal system comprises alens unit having positive refractive power and located on the extremeobject side therein.
 3. A zoom finder comprising:a system for forming animage of a measuring field frame, said system having negative refractivepower and including a semi-transparent mirror; and a vari-focal lenssystem having a lens unit having positive refractive power disposed onthe extreme object side thereof and a subsystem having negativerefractive power disposed in rear of said lens unit, said measuringfield frame forming system and said vari-focal lens system being formedin that order from the object side of the zoom finder.
 4. A zoom finderaccording to claim 3 arranged that said lens unit having negativerefractive power which is comprised in said vari-focal system is movedat the time of zooming.
 5. A zoom finder according to claim 4 whereinsaid system for forming the image of the measuring field frame comprisestwo negative lenses, and said vari-focal system comprises a lens unithaving positive refractive power which comprises two positive lenses,and a lens unit having negative refractive power which comprises twonegative lenses, and wherein the surface on the rear side of the secondnegative lens in said system for forming the image of the measuringfield frame is arranged as an aspherical surface expressed by theformula shown below when the x axis is traced in the direction of theoptical axis and the y axis is traced in the direction perpendicular tothe optical axis taking the intersecting point between the optical axisand said aspherical surface as the origin: ##EQU2## where, referencesymbol r represents the radius of curvature of the vertex portion of theaspherical surface, and reference symbol P represents the coefficient ofcone.
 6. A zoom finder according to claim 3 wherein said lens unithaving negative refractive power which is comprised in said vari-focalsystem comprises two subunits and, out of said two subunits, only thesubunit located on the object side is moved at the time of zooming.
 7. Azoom finder according to claim 3 wherein said lens unit having positiverefractive power which is comprised in said vari-focal system isarranged to comprise two subunits and arranged to be moved as a whole,at the time of zooming, by varying the airspace between said twosubunits.
 8. A zoom finder comprising:a system for forming an image of ameasuring field frame, said system having negative refractive power andincluding a semi-transparent mirror; and a vari-focal lens system havinga lens unit having positive refractive power disposed on the extremeobject side thereof, said lens unit having a chiefly vari-focal functionand a lens unit having a negative refractive power which comprises aneye piece, said measuring field frame forming system and said vari-focallens system being formed in that order from the object side of the zoomfinder.